If you placed u-boot.bak on /dev/md4 and you were using RAID-0 / LINEAR you should attach both drives to your PC and rebuild the array before mounting. (use apt-get to obtain mdadm, and use mdadm to rebuild the array)

Since the partition table is in the first sector (=512 bytes) of the harddisk, you must have made a typo. You will have to restore your partition-table and your stage-1 bootlaoder aswell as your u-boot bootloader.

Follow step 4 of THIS tutorial. Accept add "bs=512 count=200" to the command (without quotes) just to be on the safe side

If the file SDA_PTAB also contains the partition of sdx4, "sfdisk /dev/sdx < SDA_PTAB" would suffice for restoring the partition (otherwise run cfdisk/sfdisk afterwards to create sdx4 on the remaining part of the disk).